1. FIELD OF THE INVENTION
This invention relates to a process for producing a thin film resistor.
2. DESCRIPTION OF THE PRIOR ART
Heretofore, thin film resistors have been produced as shown the accompanying drawings FIG. 1(a) to FIG. 1(e). That is, a non-alkaline glass film 2 is formed on an alumina substrate 1 as shown in FIG. 1(a). Then, a tantalum film is formed on the non-alkaline glass film 2, and then said tantalum film is oxidized to form a tantalum pentoxide film 3. The tantalum pentoxide film is provided to protect the non-alkaline glass film by successive etching from attacking. A tantalum nitride film 4 is formed on the tantalum pentoxide film 3.
Successively, part of the tantalum nitride film 4 is removed by photoetching to form a pattern 4', as shown in FIG. 1(b). Only portion of the pattern 4', which is desired to be a resistor, is anodically oxidized to obtain an anodically oxidized tantalum pentoxide film 5.
Then, a Nichrome film 6a is formed on other part of pattern 4', where no tantalum pentoxide film 5 is provided, a gold film 6b on the film 6a, and a Nichrome film 6c on the film 6b, each by vapor deposition and photo-etching, to make a conductor 6, as shown in FIG. 1(c). In that case, the Nichrome film 6a serves to bond the tantalum nitride at the part of pattern 4' and the gold film 6b of good electroconductivity to each other, and the Nichrome film 6c serves to prevent diffusion of solder down to the gold film 6b when soldering is applied to solder bumps to be later formed. Of course, the Nichrome film 6c has a good adhesiveness to the gold film 6b, but a poor solder wettingness. Electrode on which solder bumps are to be formed, must be thus provided with a metal of good solder wettingness additionally.
Therefore, as shown in FIG. 1(d), a chromium film 7a is formed on part of the Nichrome film 6c of conductor 6, a rhodium film 7b on the film 7a, and a gold film 7c on the film 7b, each by masking vapor deposition, to form an electrode 7. In that case, the chromium film 7a serves to bond the rhodium film 7b to the Nichrome film 6c, and the gold film 7c serves to form solder bumps. Of course, the gold film 7c has a good adhesiveness to the rhodium film 7b. Then, the part of such a structure as shown in FIG. 1(d) is heated in the air to oxidize and stabilize the tantalum nitride film below on the anodically oxidized tantalum pentoxide film 5.
Finally, solder bumps 8 are formed on the metal film 7c as the uppermost layer of the electrode 7, as shown in FIG. 1(e), thereby producing a film resistor.
However, said process for producing the thin film resistor has the following drawbacks:
(1) The electrode is formed after the formation of the conductor, and thus the conductor and the electrode cannot be formed at the same time.
(2) Oxidation and stabilization of the tantalum nitride film are carried out in an oxidizing atmosphere, and thus a non-oxidizable, expensive gold must be used at the conductor and electrode.